This project seeks to determine the neural mechanisms for encoding species-typical communication sounds in primates. The squirrel monkey is used because of its stereotyped repertoire of calls. To test for call-selective responses, a computer is used to model call components and to manipulate these models in the frequency, amplitude and time domains. Model calls and natural calls are used as auditory stimuli to test the responses of neurons in the medial geniculate nucleus (MGN) to frequency ratios and time intervals among call components. Neural sensitivity to acoustic transient. was also tested with noise bursts. In the previous annual report, the discovery of combination-sensitive neurons in the MON was reported. These neurons respond to particular combinations of components and not to individual components. The combination-sensitive neurons in the squirrel monkey resemble those described for lower vertebrates, indicating that these neurons may be of general importance in processing species-typical communication sounds. In the past year, I have measured the call-selectivity of MON neurons and the acoustic parameters to which they are sensitive: 1. When presented with a set of natural calls that share particular call components, 10% of MGN neurons tested responded selectively to one call. This selectivity was due to a sensitivity to the temporal sequence of components. 2. The response of combination-sensitive neurons depended on the time intervals between components. critical delays ranged from 5 to 80 msec. This delay sensitivity is a mechanism by which acoustically related vocalizations may be discriminated from one another. Similar critical delays are important in phoneme identification by humans. Thus, our results may provide some insight on the neural mechanisms of human speech perception 3. Most of the MGN neurons that were unselective for vocalizations responded selectively to acoustic transient. When tested with noise bursts, these neurons responded transiently at stimulus onset and were sensitive to the absolute rate of amplitude increase. Such acoustic "edge-detectors" are analogous to the contrast sensitive cells in the visual system. 4. Preliminary results suggest that the call-selective neurons are located dorsally in the MGN, whereas the neurons that are non-selective for calls are located ventrally. Physiological mapping and anatomical tracing studies are planned to determine the functional organization of the MGN.